1. Field of the Invention
The present invention relates to a vibration driven motor for generating a travelling vibration wave on a vibration member by applying a voltage to an electromechanical energy conversion element, and for causing a relative movement between the vibration member and a member contacting the vibration member by frictional driving and, more particularly, to a high-precision type vibration driven motor.
2. Related Background Art
FIG. 4A is a plan view showing an electrode arrangement of a conventional vibration driven motor, and FIG. 4B is an exploded side view of a stator. As shown in FIGS. 4A and 4B, a common electrode surface of a thin annular electromechanical energy conversion element (e.g., a piezo-electric element) 101 is fixed to a vibration member 102 formed of an elastic member so as to constitute a stator. Projections for increasing the moving speed of a movable member and improving driving efficiency are arranged on the contact surface of the vibration member 102 with the movable member at equal intervals over the entire periphery at a rate of a plurality of projections per .lambda./2. Note that .lambda. is the wavelength of a vibration generated on the vibration member 102.
Electrodes on the other surface of the piezo-electric element 101 include an A driving electrode group (A.sub.1 to A.sub.8) and a B driving electrode group (B.sub.1 to B.sub.8) polarized at a .lambda./2 pitch so as to alternately have opposing expansion/contraction polarities with respect to the wavelength (.lambda.) of a vibration wave to be excited, and a .lambda./4-pitch vibration detection electrode S and two power supply ground electrodes G arranged between the A and B electrode groups.
When an AC voltage is applied to one of the A or B electrode groups, a standing wave having the wavelength .lambda., in which the central point of the electrode group and points separated from the central point at .lambda./2 intervals correspond to loop positions, and the central points of the loop positions correspond to node positions, is generated over the entire periphery of the vibration member 102. When an AC voltage is applied to only the other electrode group, a standing wave is similarly generated. In this case, the loop and node positions are shifted by .lambda./4 from the above-mentioned standing wave.
When AC voltages having the same frequency and a time phase difference=.pi./2 therebetween are simultaneously applied to the two electrode groups, two standing waves generated by the two groups are synthesized, and a progressive wave having the wavelength .lambda. of a flexural vibration propagating in the circumferential direction is generated on the vibration member 102. Thus, the distal ends of the projections, i.e., the contact surface of the vibration member 102 with the movable member drives a known movable member (not shown in FIGS. 4A and 4B) urged thereagainst.
In this conventional vibration driven motor, a driving circuit, in which one vibration detection electrode S is arranged in addition to the A and B driving electrode groups, and the frequency of the AC voltages to be applied to the driving electrode groups is automatically set to be a resonance frequency according to the detection output, thereby efficiently driving the vibration driven motor, is disclosed in Japanese Laid-Open Patent Application No. 61-157276.
Note that the positional relationship of the plurality of projections (five projections in FIG. 4B) per .lambda./2 of the vibration member 102 is not specified with respect to the electrode arrangement of the piezo-electric element 101 shown in FIG. 4B.
A vibration driven motor assembled with the conventional stator has high-output type performance, i.e., a rated rotation speed of 100 rpm, a torque of 4 kg.multidot.cm, and a rated output of 4 W or more. When this vibration driven motor was driven by a low output, e.g., at a rotation speed of 33.33 rpm and a torque of 1 kg.multidot.cm, required rotation nonuniformity precision, i.e., a required wow & flutter value of rotation could not be obtained, and it was found that rotation precision must be considerably improved.
It was also found that rotation precision varies depending on rotation directions.
Furthermore, it was confirmed that rotation precision varies depending on a difference in individual stators.